infrared transmittion

[ryanleung]

I want to use the IR diode to transmit the modulation signal to the receiver.  I constructed this circuit and connect two IR Diode on the output(Vmod).    I tested the IR diode using CRO, there has the modulation signal.  But the receiver can not receive the signal.  Is this circuit has any problem?
thx.....

carrier freq.:40kHz
Audio: 4kHz

View attachment 38559

 

[audioguru2]

Hi Ryan,
Do you want to amplitude-modulate the carrier? Your circuit doesn't modulate, it just mixes the signals together.
Also, your 4kHz modulation frequency is reduced by the attenuator of R2, C3 and the very low impedance of the transistor's emitter.

 

[ryanleung]

the output is like this figure, can i transmit this signal to the receiver?

View attachment 38560

 

[audioguru2]

Hi Ryan,
Your signal looks like AM modulation but I think it would have just the two frequencies mixed together instead of modulation and would look like this.
AM modulators I have seen change the supply voltage of the carrier oscillator like this one.

View attachment 38562

View attachment 38563

 

[ryanleung]

but this signal can transmit to the receiver using infrared???

 

[audioguru2]

IR can be directly modulated with AM or FM audio. Frequently high frequency subcarriers like 100kHz and 200kHz are used then FM modulated for stereo.